ci: Make Cirrus CI Windows build with --enable-werror

Author: hebasto

ci/test/00_setup_env_win64.sh

@@ -14,7 +14,3 @@ export PACKAGES="python3 nsis g++-mingw-w64-x86-64 wine-binfmt wine64 wine32 fil
 export RUN_FUNCTIONAL_TESTS=false
 export GOAL="deploy"
 export BITCOIN_CONFIG="--enable-reduce-exports --disable-gui-tests --disable-external-signer"
-
-# Compiler for MinGW-w64 causes false -Wreturn-type warning.
-# See https://sourceforge.net/p/mingw-w64/bugs/306/
-export NO_WERROR=1